A purser is a crew member responsible for handling money and goods aboard a ship. This seemed to be the closest thing I could find to someone being responsible for choosing what cargo to acquire and buy, and then to sell it at the destination port. Probably because the typical "free trader" model of cargo business - buying cargo and then selling it elsewhere at a profit - doesn't exist much in the real world. Almost all cargo shipping is done on contract, when someone who owns the cargo hires a ship to move it from one place to another. At no point does the ship's crew own the cargo.

I've seen the "free trader" system also go by the name of "speculative cargo", but a search doesn't bring up any Wikipedia page or even anything much about the practice in the real world except for a few odd cases of people shipping booze to colonial Australia.

Anyway, the point is that there doesn't seem to be a real world name for a ship's officer who is in charge of buying cargo and selling it, because it actually doesn't happen much in reality. There is the Chief mate, who acts as "cargo officer", but this role is purely looking after the physical operations of loading, stowage, securing, and unloading of cargo, not the transactional operations of buying and selling it.

So I chose "purser" as the best thing to describe Serron's role.

EDIT: Several readers pointed me at this: supercargo. With a slight twist of interpretation, this fairly accurately describes Serron's role on board the Legacy.